Both Float and BigDecimal do respond to infinite? but Integer derivatives don't. It sort of makes sense, since we don't have an infinity value for Fixnum/Bignum. However, it makes polymorphic code hard.

Another real-world use-case where I found Numeric#infinite? useful is JSON validation. Infinities are not allowed in JSON so a programmer want to filter them out before converting a Numeric into JSON, but that is not straight-forward right now. With this method it becomes much OO-ish.